The present invention relates to a curable composition, and more particularly to a curable composition which less suffers from foaming at the time of curing, and is capable of producing a cured product which is white and has such good weathering resistance that can prevent occurrences of surficial cracks even when the cured product is exposed outdoors, and which curable composition needs small amounts of plasticizers, thereby producing a cured product which is suitable for sealing materials because it suffers from so little transfer of plasticizers into a paint coating applied thereonto that it avoids the softening of the paint coating.
When polymers containing two or more thiol groups in one molecule are mixed with an oxidizing agent, they are easily cured, and their thiol groups are easily reacted with epoxy groups, isocyanate groups, etc. to become high-molecular compounds. Accordingly, such polymers are widely used for sealing materials, paints, adhesives, etc.
Since this polysulfide polymer has a structure of polysulfide bonds S.sub.x (x=1-5) in a molecule, it shows high polarity, and only limited types of plasticizers can be used. Since widely used, inexpensive plasticizers cannot be used, compositions such as sealants containing the polysulfide polymer are relatively expensive. In addition, since it shows a higher viscosity than expected from considering its molecular weight, a larger amount of a plasticizer is required to be added to improve the handling of the composition at a curing temperature. Accordingly, when a cured product of a sealant produced from the polysulfide polymer is covered by a paint coating, plasticizers tend to exude, thereby softening the paint coating.
Japanese Patent Laid-Open No. 3-134058 discloses a polymer composition comprising a polysulfide polymer and a thiol group-containing polyether. This composition is compatible with widely used, inexpensive plasticizers and suitable for sealing materials. Further, it only needs small amounts of plasticizer. Japanese Patent Laid-Open No. 4-7331 discloses a polysulfide polymer which is also compatible with widely used, inexpensive plasticizers and only needs small amounts of plasticizer. Further, since this polysulfide polymer contains reduced amounts of low-molecular weight components, a cured product obtained by curing it with an oxidizing agent, etc. shows improved elongation. In addition, when a paint is applied to a cured product of a sealant produced from these polymers, the paint coating is not likely to be softened by the transference of plasticizers.
However, since an oxidizing agent having a large oxidizing power such as PbO.sub.2, MnO.sub.2, etc. is used as a curing agent for each of a thiol group-containing polysulfide polymer and polysulfide polyether, it is inevitable for them to be colored by the oxidizing agent, thereby failing to provide a white cured product. Further, since the reaction between inorganic oxidizing agents such as PbO.sub.2, MnO.sub.2, etc. and polysulfide polymer is a solid-liquid reaction, it is difficult to control the curing rate.
Since a sealant of a polyurethane prepolymer produced by reacting a polyether polyol with a polyisocyanate compound has good compatibility with widely used plasticizers and also has low viscosity, it is unlikely that a paint coating applied to a cured product of the polyurethane prepolymer is softened. However, this polyurethane prepolymer has such a problem that when it is cured with or without a polyol and a catalyst under the conditions of high temperatures and humidities, it tends to suffer from foaming.